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As explained in Section “Reflecting Surfaces”, a skybox can be thought of as an infinitely large, textured box that surrounds a scene. Sometimes, skyboxes (or skydomes) are implemented by sufficiently large textured models, which approximate an infinitely large box (or dome). However, Section “Reflecting Surfaces” introduced the concept of a cube map, which actually represents an infinitely large box; thus, we don't need the approximation of a box or a dome of limited size. Instead, we can render any screen-filling model (it doesn't matter whether it is a box, a dome, or an apple tree as long as it covers the whole background), compute the view vector from the camera to the rasterized surface point in the vertex shader (as we did in Section “Reflecting Surfaces”) and then perform a lookup in the cube map with this view vector (instead of the reflected view vector in Section “Reflecting Surfaces”) in the fragment shader:

For best performance we should, of course, render a model with only a few vertices and each pixel should be rasterized only once. Thus, rendering the inside of a cube that surrounds the camera (or the whole scene) is fine.

The shader should be attached to a material, which should be attached to a cube that surrounds the camera. In the shader code, we deactivate writing to the depth buffer with ZWrite Off such that no objects are occluded by the skybox. (See the description of the depth test in Section “Per-Fragment Operations”.) Front-face culling is activated with Cull Front such that only the “inside” of the cube is rasterized. (See Section “Cutaways”.) The line Tags { "Queue" = "Background" } instructs Unity to render this pass before other objects are rendered.

Shader"GLSLshaderforskybox"{Properties{_Cube("EnvironmentMap",Cube)=""{}}SubShader{Tags{"Queue"="Background"}Pass{ZWriteOffCullFrontGLSLPROGRAM// User-specified uniformuniformsamplerCube_Cube;// The following built-in uniforms // are also defined in "UnityCG.glslinc", // i.e. one could #include "UnityCG.glslinc" uniformvec3_WorldSpaceCameraPos;// camera position in world spaceuniformmat4_Object2World;// model matrix// Varyingvaryingvec3viewDirection;#ifdefVERTEXvoidmain(){mat4modelMatrix=_Object2World;viewDirection=vec3(modelMatrix*gl_Vertex-vec4(_WorldSpaceCameraPos,1.0));gl_Position=gl_ModelViewProjectionMatrix*gl_Vertex;}#endif#ifdefFRAGMENTvoidmain(){gl_FragColor=textureCube(_Cube,viewDirection);}#endifENDGLSL}}}

The shader above illustrates how to render a skybox by rendering a cube around the camera with a specific shader. This is a very general approach. Unity, however, has its own skybox system that doesn't require any game object: you just specify the material with the skybox shader in the main menu Edit > Render Settings > Skybox Material and Unity takes care of the rest. Unfortunately, we cannot use our shader for this system since we have to do the lookup in the cube texture at the position that is specified by the vertex texture coordinates. This is actually easier than computing the view direction. Here is the code: